I found you need two things for the recommended RTC udp port forwarding solution to work:
1. Port forwarding UDP/8443 in firewall to the Unraid/Mattermost server
2. In the Unraid docker deployment page for Mattermost, manually add another listening port with 8443. (bottom of page +
Add another Path, Port, Variable, Label or Device) Config Type = Port. I just named it RTC listen port and put 8443 as Container and Host port, Connection type UDP - then Add and Apply.
Make sure 8443 is not taken by another container, or you may need to change the listening port in Mattermost call app config.